March fog foxes many

All on a Monday: fog in the morning and (right) lightning before rain in the evening. Pictures by Sanjoy Chattapadhyaya and Sanjoy Ghosh

Morning: a dense curtain of winter-type fog

Afternoon: hot and humid

Evening: smart shower and a sigh of relief

Calcutta experienced three seasons in one day, the most bizarre being the Monday morning fog that engulfed the city for a couple of hours.

Met experts said the phenomenon, rare for this time of the year, was caused by the high level of moisture in the air over the city and a cold wind blowing from the northwest early in the morning. A spell of rain in the districts the day before had pumped up the moisture level in the Calcutta air.

Weather scientists attributed another reason for the high moisture content in the air: water vapour from the Bay of Bengal was accumulating over the past few days because of a network of weather systems but was not getting released in the form of rainfall.

When a cold wind from the northwest blew in on Monday from the districts and mixed with the moist air, the vapour condensed and fog was formed.

“The air was cold in the districts because it had rained there. A northwesterly wind was also blowing early in the morning,” said Gokul Chandra Debnath, the director of the India Meteorological Department, Calcutta.

The fog brought down runway visibility at Calcutta airport to 200m, where 1,000m is the standard for this time of the year. “Thick fog had covered the runway and visibility was down for more than an hour. It’s extremely unusual,” a senior airport official said.

“We had fog till late February but not the fag end of March,” he added.

Low visibility from around 5.30am to about 6.45am affected flight operations. A Jet Airways Mumbai-Calcutta flight that was scheduled to land at 5.30am had to be diverted to Dhaka, officials said. It returned a few hours later.

Several flights from Calcutta were delayed by 15 to 20 minutes in the morning. Calcutta airport has Category II Instrument Landing System that allows flights to operate at a minimum visibility of 350m.

The weather kept morning walkers at Dhakuria lake indoors. “I woke up and found I could hardly see a thing beyond a few feet. So I decided not to go for the morning walk,” said Shyamal Mukherjee, a resident of Dover Lane.

The fog dissipated later in the day when the sun came out and heated up the air. The heated air formed a tall cloud and it rained in the evening.